1. Field of the Invention
This invention relates to an optical module that connects a catheter to a processor in a catheter oximetry system, and more particularly, it relates to optical modules of the readily portable type that can remain with the inserted catheter and the patient as the patient is transferred from one location to another.
2. Description of the Prior Art
A catheter oximetry system provides accurate, continuous, real-time measurement of mixed venous oxygen saturation using multiple wavelength reflection spectrophotometry. The color of red blood cells progressively changes from scarlet to purple as the amount of oxygen that the red blood cells are carrying decreases. When light of different selected wavelengths illuminates the blood, the amount of light backscattered, or reflected, at each wavelength depends upon the color, and therefore, oxygen level of the blood. Careful choice of wavelengths in the transmittal light allows accurate measurement of oxygenated hemoglobin with minimal interference by other blood characteristics such as temperature, pH, and hematocrit.
Approximately 98% of the oxygen in the blood is chemically combined with hemoglobin in red blood cells. The absorption of red and infrared light is substantially different for oxygenated and deoxygenated hemoglobin, and it varies for different wavelengths of light within this red/infrared spectrum. Therefore, the relative amounts of oxygenated hemoglobin and deoxygenated hemoglobin in the blood can be determined by measuring the relative absorption of light at different selected wavelengths. The percentage of hemoglobin which is in the oxygenated form is defined as the oxygen saturation of the blood in the equation: ##EQU1## where HbO.sub.2 is the oxygenated hemoglobin concentration and Hb is the deoxygenated hemoglobin concentration.
A widely used catheter oximetry system consists of three basic components: (1) a disposable fiberoptic pulmonary artery catheter that has a distal end adapted to be inserted into a vein or artery of a patient and that interfaces at its other end with (2) an optical module containing light-emitting diodes, a photodetector and associated electronics which, in turn, interfaces with the electrical leads of (3) a computer-based instrument that performs all of the data processing and control functions with displays, alarms and associated read-out devices.
The optical module in the aforedescribed system thus plays the important part of providing the electro-optical connection between the processor, where all of the electronic processing and computations are carried out, and the catheter, which serves as a transmitting guide for the individual light pulses and a receiving guide for the light backscattered (reflected) from the patient's blood. The optical module is comprised of an enclosure or housing having therewithin a plurality of (e.g., three) light-emitting diodes to provide discrete light sources at the selected wavelengths available for performing the oxygen saturation measurements. Light from each of the LED sources is sequentially transmitted in short pulses under the control of electrical signals from the processing apparatus through a transmitting light guide at the connector end of the catheter to illuminate the blood flowing past the catheter tip at the other end. This illuminating light is absorbed, refracted and reflected by the blood, and a portion of the reflected light is collected by the aperture of a second receiving light guide at the catheter tip. This collected light is returned through the catheter to a photodetector in the optical module. The photodetector converts these received light signals to electrical signals which are amplified and transmitted to the processing apparatus over the electrical connections to the module. Using the relative intensities of the signals representing the light levels at the various different wavelengths, the processor calculates oxygen saturation and outputs this information to the user.
Because the actual light levels which can be collected from the backscattered light in the blood are very low, and because the differences in the relative intensities at the different wavelengths are small, variables in the optical system are extremely critical, and the optical module/catheter combination must be carefully calibrated each time it is used so that it can be normalized to some standard in order to provide usable output readings. For example, differences in the transmitting properties of each of the light guides and differences in signal output of the LED's can significantly affect the relative signal levels and hence the end calculations.
In order to accommodate the foregoing problem, it is conventional to first make a calibration reading of the catheter by attaching it to a processor/optical module combination and generating light signals into and receiving reflected light signals back from some known standard reference material as, for example, a reference block which might be packaged with the catheter. This information then remains with the processor and is used to adjust the output readings it receives from the optical module in order to provide accurate and usable data for the doctor or nurse monitoring the patient's blood condition. However, it frequently is the case that a patient may be moved from one area of the hospital to another, e.g., from the operating room to a recovery room or from a recovery room to a hospital room, with the catheter/optical module combination remaining with the patient but with different processors being utilized with such combination at each of the different locations. When this occurs, a totally new calibration must be obtained since the new processor will not have the relevant calibration information, thus hindering the flexibility of use of the oximetry system and adding to the total time and cost of hospital care.